Re: [CFT][PATCH 00/10] Making new mounts of proc and sysfs as safe as bind mounts (take 2)
From: Eric W. Biederman <hidden>
Date: 2015-05-28 15:03:28
Also in:
linux-fsdevel
Subsystem:
the rest · Maintainer:
Linus Torvalds
Possibly related (same subject, not in this thread)
- 2015-06-16 · Re: [CFT][PATCH 00/10] Making new mounts of proc and sysfs as safe as bind mounts (take 2) · Richard Weinberger <hidden>
- 2015-06-16 · Re: [CFT][PATCH 00/10] Making new mounts of proc and sysfs as safe as bind mounts (take 2) · Daniel P. Berrange <hidden>
- 2015-06-16 · Re: [CFT][PATCH 00/10] Making new mounts of proc and sysfs as safe as bind mounts (take 2) · Daniel P. Berrange <hidden>
- 2015-06-16 · Re: [CFT][PATCH 00/10] Making new mounts of proc and sysfs as safe as bind mounts (take 2) · Daniel P. Berrange <hidden>
- 2015-06-06 · Re: [CFT][PATCH 00/10] Making new mounts of proc and sysfs as safe as bind mounts (take 2) · Eric W. Biederman <hidden>
Serge Hallyn [off-list ref] writes:
Quoting Andy Lutomirski (luto-kltTT9wpgjJwATOyAt5JVQ@public.gmane.org):quoted
On Fri, May 22, 2015 at 10:39 AM, Eric W. Biederman [off-list ref] wrote:quoted
I had hoped to get some Tested-By's on that patch series.Sorry, I've been totally swamped. I suspect that Sandstorm is okay, but I haven't had a chance to test it for real. Sandstorm makes only limited use of proc and sysfs in containers, but I'll see if I can test it for real this weekend.Testing this with unprivileged containers, I get lxc-start: conf.c: lxc_mount_auto_mounts: 808 Operation not permitted - error mounting sysfs on /usr/lib/x86_64-linux-gnu/lxc/sys/devices/virtual/net flags 0
Grr.. I was afraid this would break something. :( Looking at my system I see that sysfs is currently mounted "nosuid,nodev,noexec" Looking at the lxc-start code I don't see it as including any of those mount options. In practice for sysfs I think those options are meaningless (as there should be no devices and nothing executable in sysfs) but I can understand the past concerns with chmod on virtual filesystems that would incline people to use them, so I think the failure is reporting a legitimate security issue in the lxc userspace code where the the unprivileged code is currently attempting to give greater access to sysfs than was given by the original mount of sysfs. As nosuid,nodev,noexec should not impair the operation of sysfs operation it looks like you can always specify those options and just make this concern go away. Something like the untested patch below I expect.
diff --git a/src/lxc/conf.c b/src/lxc/conf.c
index 9870455b3cae..d9ccd03afe68 100644
--- a/src/lxc/conf.c
+++ b/src/lxc/conf.c@@ -770,8 +770,8 @@ static int lxc_mount_auto_mounts(struct lxc_conf *conf, int flags, struct lxc_ha { LXC_AUTO_PROC_MASK, LXC_AUTO_PROC_MIXED, "%r/proc/sysrq-trigger", "%r/proc/sysrq-trigger", NULL, MS_BIND, NULL }, { LXC_AUTO_PROC_MASK, LXC_AUTO_PROC_MIXED, NULL, "%r/proc/sysrq-trigger", NULL, MS_REMOUNT|MS_BIND|MS_RDONLY, NULL }, { LXC_AUTO_PROC_MASK, LXC_AUTO_PROC_RW, "proc", "%r/proc", "proc", MS_NODEV|MS_NOEXEC|MS_NOSUID, NULL }, - { LXC_AUTO_SYS_MASK, LXC_AUTO_SYS_RW, "sysfs", "%r/sys", "sysfs", 0, NULL }, - { LXC_AUTO_SYS_MASK, LXC_AUTO_SYS_RO, "sysfs", "%r/sys", "sysfs", MS_RDONLY, NULL }, + { LXC_AUTO_SYS_MASK, LXC_AUTO_SYS_RW, "sysfs", "%r/sys", "sysfs", MS_NODEV|MS_NOEXEC|MS_NOSUID, NULL }, + { LXC_AUTO_SYS_MASK, LXC_AUTO_SYS_RO, "sysfs", "%r/sys", "sysfs", MS_NODEV|MS_NOEXEC|MS_NOSUID|MS_RDONLY, NULL }, { LXC_AUTO_SYS_MASK, LXC_AUTO_SYS_MIXED, "sysfs", "%r/sys", "sysfs", MS_NODEV|MS_NOEXEC|MS_NOSUID, NULL }, { LXC_AUTO_SYS_MASK, LXC_AUTO_SYS_MIXED, "%r/sys", "%r/sys", NULL, MS_BIND, NULL }, { LXC_AUTO_SYS_MASK, LXC_AUTO_SYS_MIXED, NULL, "%r/sys", NULL, MS_REMOUNT|MS_BIND|MS_RDONLY, NULL },
Alternately you can read the flags off of the original mount of proc or sysfs.
diff --git a/src/lxc/conf.c b/src/lxc/conf.c
index 9870455b3cae..50ea49973e80 100644
--- a/src/lxc/conf.c
+++ b/src/lxc/conf.c@@ -712,7 +712,9 @@ static unsigned long add_required_remount_flags(const char *s, const char *d, struct statvfs sb; unsigned long required_flags = 0; - if (!(flags & MS_REMOUNT)) + if (!(flags & MS_REMOUNT) && + (strcmp(s, "proc") != 0) && + (strcmp(s, "sysfs") != 0)) return flags; if (!s)
Eric